Tissue morphogenesis and homeostasis require proper stem cell (SC) function. Our long-term goal is to understand the molecular mechanisms that underlie how SCs self-renew, maintain their multipotent state, and differentiate into multiple lineages. This knowledge of SC biology is important to better manipulate SCs for uses in regenerative medicine, and to understand how their aberrations might contribute to a variety of diseases including tumorigenesis. 2-catenin is the key mediator of the canonical Wnt signaling pathway, which controls many aspects of development, including SC activation during hair morphogenesis and regeneration. Tcf3 and Tcf4 are HMG transcription factor members that either can activate gene transcription when bound to 2-catenin, or can repress target gene transcription when bound to corepressors. However, it remains unclear whether, in the absence of 2-catenin, Tcf3/4 repress only Wnt target genes or whether Tcf3/4 also function to repress genes that are independent of the Wnt/2-catenin signaling pathway. Tcf3 has been shown to be one of the core components of the regulatory network of embryonic SCs, and its expression in other adult SCs suggests that it likely plays an important role in these SCs as well. Our preliminary results show that Tcf3/4 loss affects skin epithelial stem cell homeostasis. In this proposal we aim to determine the Wnt-dependent and Wnt-independent function of Tcf3 and Tcf4 in maintaining skin epithelial stem cell homeostasis. We employ a genetic mouse model with targeted Tcf3/4 genes to dissect the mechanisms by which Tcf3/4 regulate stem cells homeostasis, using both in vitro and in vivo assays. This will allow us to delineate the Wnt/2-catenin dependent and independent role of Tcf3/4 in controlling specific function that allow epidermal stem cells homeostasis. PUBLIC HEALTH RELEVANCE: Understanding how stem cells grow and are controlled is of tremendous interest because of the potential use of stem cells for tissue replacement. In addition, it is also important for our understanding of cancer since cancer cells often display many characteristics of stem cells, as recent evidence suggests that cancerous cells might arise from stem cell origin. In this proposal we propose to determine how the loss of two particular genes disrupts the maintenance of skin epithelial stem cells and identify the mechanism by which they control their fate.